Enable sync for the settings from the Extension Settings API.

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index f45b0f5e6508df63f7ee74ffeb0a49bb029e7bd0..779030ac8c87c3e53dde301d93e54ec9387e0d3d 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -503,6 +503,10 @@ const char kEnableShortcutsProvider[]       = "enable-shortcuts-provider";
 
 // On platforms that support it, enable smooth scroll animation.
 const char kEnableSmoothScrolling[]         = "enable-smooth-scrolling";
+
+// Enable syncing extension settings.
+const char kEnableSyncExtensionSettings[]   = "enable-sync-extension-settings";
+
 // Enable OAuth sign-in for sync.
 const char kEnableSyncOAuth[]               = "enable-sync-oauth";